Bullet run out how much matters?

[One Shot One Thrill]

Simple question I ask?
How much bullet run out does it take to make a difference? I am not interested in theory and the obvious answer only in answers that have been tested and proven.

 

[Texas10]

I'm certainly no expert but don't think that there is one answer that covers all bullet and barrel combinations. I have had bullet seating errors on the loading bench, marked as a sighter and watched it shoot right into the groups.

 

[gme]

This lot of ammo had up to .003 runout

 

[Bart B.]

No more than 1% of bullet diameter.

Index the round in the chamber with its marked high point at high noon; works well for rounds with several thousandths runout.

Depending on where the cartridge points are it rests at, the same round can show twice the runout with different gauge setups.

 

[Zero333]

I've compared 0.000" vs 0.003" runout at 300 meters and could not tell the difference.

I'd like to test 0.005" runout but my loads never get above 0.003" and i don't feel like artificially inducing more runout.

 

[Uncle Ed]

The military considers match grade ammo to have .003 or less runout. That being said a full length resized case gives you more wiggle room for bullet alignment in the throat.

Below German Salazar is answering a question about partial full length resizing vs full length resizing and bullet alignment in the throat.

"a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway.

In conclusion, I believe that allowing the bullet to find a relatively stress-free alignment in the throat by full length sizing (including the neck) and turning necks to enhance concentricity gives the bullet the best probability of a well-aligned start into the rifling."

The Rifleman's Journal
Germán A. Salazar

 

[Bart B.]

This is good information......

I have both gauges below and they hold and rotate the case in different ways. And the Hornady gauge care how much coffee you drink and how much your hand shakes.

For simplicity sake the Hornady gauge holds the case like a full length size case would be held in the chamber. Meaning the rear of the case held in the recessed bolt face and by the bullet in the throat, and the body of the case not touching the chamber walls.

With gauges that spin on the case body and if the case body is egg shaped you will have more indicated runout. I think of these type gauges better for neck sized cases where the case body can have a effect on bullet alignment. Meaning if you have unequal case wall thickness the case can bulge on the thin side and become egg shaped.

Bottom line, practice finger Yoga until your spinning finger reaches Zen and then you will have a total state of rotational enlightenment.

 

[Bart B.]

The military considers match grade ammo to have .003 or less runout.

Good idea.

Don't know how long that's been a spec. Never seen it listed as a spec.

I've measured 7.62 M118 and M852 lots with 25% or more rounds having .004" to .006" runout.

No commercial runout gauge holds cartridges headspacing on their shoulder for runout measuring like they're held in the chamber; so thinks me.

 

[Dusty Stevens]

Strive for zero then open your calipers .003 and look at that gap. Go have fun

 

[MagnumManiac]

One question; how much misalignment in the chamber does a plunger ejector cause from base to bullet tip?

I have posted this before, even when I ‘made’ cartridges with .008” runout, after chambering them, they had reduced to .0025”-.003”. The chamber throat in question is tighter than SAAMI, but not a huge difference.
I fired those and other with runout from .0005”-.008”, not a single difference could be seen at 300 or 600, the groups at 900 showed neck tension variance due to the induced runout that I did to them, it obviously changes the tension when the bullet moves in the neck, no fliers were recorded that were outside the norm for 20 shot strings.

I don’t even check runout anymore, my conentricity gauge gathers dust.

Cheers.

 

[dkhunt14]

I think the answer to your question depends if the bullet is in the lands or jumping. I have shot some very bad runout cases in the lands at 1000, they shot 10 shot 4 inch groups. Matt

 

[Intheshop]

Cast bullets here at starting JB velocity.

Two things that immediately showed up on paper was/are.... annealing and getting deadnuts straight ammo. The former is harder to quantify because if you're getting good performance and repeatability withOUT annealing.... cool. Hard to argue. What's easier to explain,because it shows up so much better on a cast bullet.... is runout. The launch cycle of cast has to be controlled to the umpteenth degree. Ya'll think bench manners matter shooting JB's..... try that devided by ten. Things like screw torque, depending on the rig,may or may not show up on paper..... with cast it's a heck of a lot more apparent. So,the same for runout.

A well behaved rig/load/tune that will hold it's own in the wind..... these are med hard jams. It's dang obvious on paper what straight ammo will produce. It gets lost in the noise with jacketed.... best I can tell. It's more dependent on a particular rig than any blanket statement can be proved or not.

 

[Bart B.]

One question; how much misalignment in the chamber does a plunger ejector cause from base to bullet tip?

Most people forget or don't know the rimless bottleneck case shoulder centers in the chamber shoulder from firing pin impact before the primer fires, whatever misalignment of the case neck has to case shoulder is transferred to the bullet. If the case body is smaller than the chamber, no part of the case body touches the chamber; except at its pressure ring where extractors push it against the chamber. A couple thousandths head clearance is normal.

A perfectly straight 3" long cartridge with 2" headspace, .002" head clearance and its pressure ring is .002" diameter smaller than chamber will have its bullet tip. 001" off center in the bore when it fires, assuming no interference by the throat. Several such rounds the same will position the same when fired. Plunger ejector's may not push case heads off center but usually push rounds forward to the limits of the chamber centering the front of the cartridge in the chamber. 2 ounce firing pins striking primers at near 10 fps will move the 2 ounce cartridge before it fires if there's shoulder clearance.

Seldom is this mentioned in print. Salazar's comments above don't. It's easy to figure out

 

[JimSC]

? I am not interested in theory and the obvious answer only in answers that have been tested and proven.

Seems like actual tests of runout and it's effect on paper is like annealing. Everyone "knows" it is necessary and the more expensive equipment you have the better you will "cure" the "problem" but no one can be bothered to actually test the theory.

 

[Bart B.]

.. ...but no one can be bothered to actually test the theory.

I have watched case mouths and necks center in chambers from firing pins pushing on primers in case heads.

And watched case mouths move in chamber mouths opposite that of case pressure rings.

The barrel was cut off at the chamber mouth first.

 

[Preacher]

This is what I came up with a while ago..

 

[JFrank]

This is what I came up with a while ago..
View attachment 1080037

Very nice"
Were you able to duplicate this test?

 

[JFrank]

I have watched case mouths and necks center in chambers from firing pins pushing on primers in case heads.

And watched case mouths move in chamber mouths opposite that of case pressure rings.

The barrel was cut off at the chamber mouth first.

May I ask
How are you able to watch case mouths move in a chamber?

 

[DirtySteve]

Runout causes yawing around the center of the bullets axis. The speed of the bullet and rate of twist determine how much runout will affect the accuracy. (Properly stabilized according to Litz is a factor of 1.5 or better.)

"Bullets that are properly stabilized will go to sleep quickly, without affecting flight performance (BC). If launched with adequate gyroscopic stability, they will fly with near zero pitching and yawing motion, and exhibit equal drag and BC from any rifle they’re fired from."

Litz, Bryan. Applied Ballistics For Long-Range Shooting 3rd Edition: Understanding the Elements and Application of External Ballistics for Successful Long-Range Target Shooting and Hunting (Kindle Locations 2640-2642). Applied Ballistics, LLC. Kindle Edition.

 

[Bart B.]

May I ask
How are you able to watch case mouths move in a chamber?

Cut the barrel off at the chamber mouth first.

Used dead primed 308 cases in a "chambered" action, very straight cases with headspace a few thousandths less than chamber. With the extractor and striker spring removed, the case mouth would rest off center in the chamber mouth until the striker/firing-pin, or in line ejector, pushed the case forward centering its shoulder in the chamber shoulder. Case mouth went from off center to dead center or a smidgen opposite case head. Couple thousandths uniform clearance around case necks and mouths. Used a 10X loupe to easily see details.